Radiotherapy uses high doses of ionizing radiation to damage cancer cell DNA, preventing them from dividing. It treats ~50% of all cancer patients at some point in their care — either with curative or palliative intent.
Understanding why we use fractionated (divided) doses over multiple days requires understanding how radiation affects cells — both tumor cells and normal tissue. The four R's explain how fraction timing improves tumor control while protecting normal tissues.
1. Repair
The image shows daily radiation doses injuring both normal and tumor cells, but normal cells repair sublethal DNA damage during the hours to days between fractions. Tumor cells repair less effectively, so damage accumulates across treatments.
Clinical point: fractionation exploits the repair advantage of normal tissue, improving the therapeutic ratio.
2. Redistribution / Reassortment
The image maps tumor cells across the cell cycle before treatment and after repeated daily fractions. Late S-phase cells are more resistant, while mitotic and G2 cells are more sensitive; repeated fractions gradually catch more cells in vulnerable phases.
Clinical point: spacing fractions gives tumor populations time to reassort into phases where radiation is more effective.
3. Reoxygenation
The image contrasts an oxygen-poor tumor core during the first fraction with improved oxygen delivery before the next fraction. As outer tumor cells die and blood flow reaches deeper cells, previously hypoxic cells become reoxygenated and more radiosensitive.
Clinical point: oxygen fixes radiation-induced DNA damage, so reoxygenation makes later fractions more lethal to the tumor.
4. Repopulation
The image shows both tumor cells and normal mucosa or skin cells dividing between fractions. Normal tissue repopulation helps healing, but tumor repopulation can reduce control if the overall treatment course is stretched too long.
Clinical point: accelerated schedules may be used to limit tumor repopulation while still allowing normal tissue recovery.
Labeled components of a modern medical linear accelerator used for external beam radiotherapy.
The linear accelerator (LINAC) is the standard machine for external beam radiotherapy. The labeled image identifies the major treatment and control components.
A modern LINAC delivers high-energy X-rays (6–18 MV) or electrons (4–22 MeV) to the tumor from multiple gantry angles, with the tumor at the isocenter. Sophisticated shaping of the beam (MLC, IMRT, VMAT) maximizes dose to tumor while minimizing dose to surrounding normal tissue.
Radiotherapy has evolved from simple opposed beams to highly conformal, motion-adapted delivery — dramatically improving the therapeutic ratio.
CT-based treatment planning with beams shaped by collimators/MLCs to conform to the 3D shape of the target. The modern baseline for external beam RT. Multiple fixed beam angles from 3D planning CT.
MLC leaves move during delivery, creating non-uniform (modulated) fluence within each beam. Allows "dose painting" — high dose to tumor, steep dose gradient to organs at risk. Step-and-shoot or sliding-window techniques.
Gantry rotates continuously while MLC positions, dose rate, and gantry speed all vary simultaneously. Highly conformal delivery in 1–3 arcs. Often faster than IMRT. RapidArc® (Varian) is a proprietary VMAT system.
Very high dose per fraction (>5 Gy/fx) in 3–5 fractions. Precise targeting of small tumors (lung, liver, spine, prostate). Requires sub-millimeter accuracy and immobilization. Requires IGRT and motion management.
Single high-dose treatment (15–25 Gy) for intracranial targets. GammaKnife® uses ~200 Co-60 sources; Linac-based SRS uses multiple micro-arcs. Used for brain mets, AVM, acoustic neuroma, trigeminal neuralgia.
Radioactive sources placed inside or adjacent to tumor. HDR (high dose rate) — Ir-192 afterloader moves through catheters. LDR (low dose rate) — permanent seeds (I-125, Pd-103) or temporary implants. Used in prostate, cervix, breast, skin cancers.
Protons deposit most energy at the Bragg peak — abrupt stop at calculated depth with minimal exit dose. Ideal for pediatric tumors and structures adjacent to critical organs. Requires large cyclotron/synchrotron facility.
Daily imaging (kV, CBCT, ExacTrac, MRI-LINAC) to verify patient positioning and target location before each fraction. Accounts for inter-fraction variation in organ position, tumor shrinkage, and weight loss.
Radiotherapy requires meticulous planning to ensure the right dose is delivered to the right place, every fraction.
Patient positioned in treatment position (reproducible immobilization devices: masks, cradles, boards). Planning CT acquired. MRI/PET may be fused for target delineation.
Radiation oncologist delineates: GTV (gross tumor), CTV (clinical target — includes microscopic spread margin), PTV (planning target — CTV + setup uncertainty margin). Organs at risk (OARs) also outlined: spinal cord, lung, bowel, bladder, heart, etc.
Prescription: total dose (Gray) / number of fractions / dose per fraction / OAR dose constraints. Standard fractionation: ~2 Gy/day, 5 days/week. Hypofractionation: larger doses, fewer fractions (e.g., 3–5 Gy/fx for prostate). Hyperfractionation: smaller fractions, twice daily.
Medical physicist designs beam arrangement (angles, energies, weights), MLC shapes, and optimization objectives in the Treatment Planning System (TPS) (e.g., Eclipse, RayStation, Pinnacle). Dose-volume histograms (DVH) assess plan quality. Iterative optimization to meet PTV coverage and OAR constraints.
Radiation oncologist approves the plan. Medical physicist performs independent monitor unit (MU) calculation verification and patient-specific QA (e.g., IMRT QA with phantom). Patient-specific QA verifies dose delivery before first treatment.
Radiation Therapists (RTTs) set up and treat patients daily. IGRT imaging (CBCT) performed each fraction. RTTs monitor patient tolerance, document treatment, and maintain immobilization accuracy throughout the treatment course.
Radiotherapy is used in all major cancer types, either as primary treatment or combined with surgery and chemotherapy.
Whole-brain RT, focal boost post-surgery, SRS for brain mets. Concurrent temozolomide + RT for GBM (Stupp protocol). Craniospinal irradiation for medulloblastoma.
Curative SBRT for early-stage inoperable lung cancer. Concurrent chemoradiotherapy for locally advanced NSCLC. Mesothelioma palliative RT. Prophylactic cranial irradiation in SCLC.
Post-lumpectomy whole-breast RT (WBI) or partial-breast irradiation (APBI). Post-mastectomy RT (PMRT) for high-risk disease. Modern hypofractionation: 40 Gy/15 fractions replaces 50 Gy/25 fractions.
Prostate: definitive EBRT or brachytherapy. Cervix: concurrent cisplatin + EBRT + brachytherapy (curative). Rectal: neoadjuvant CRT pre-surgery. Bladder: bladder-preservation trimodality therapy.
Nasopharyngeal, oropharyngeal, laryngeal cancers. IMRT for salivary gland sparing. Concurrent platinum-based chemoradiotherapy. Organ preservation for larynx cancer.
Bone metastases (8 Gy single fraction for pain relief). Brain metastases (WBRT or SRS). Spinal cord compression (urgent). Obstruction/bleeding palliation. SBRT for oligometastatic disease.
The Radiation Therapist's (RTT's) role: RTTs are the primary clinicians responsible for daily treatment delivery. They position patients with reproducibility, operate the LINAC, perform and review IGRT imaging, manage immobilization devices, monitor and document patient toxicity, and communicate with the multidisciplinary team throughout the treatment course. It is a technically demanding, patient-focused clinical role at the intersection of physics, technology, and care.
You've explored all 6 imaging modalities plus the history of radiology. Review any section, or go back to compare modalities side by side.
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